Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State-of-the-art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, most epoxy formulations absorb moisture which reduces their high temperature properties. As a result they are not suitable for use at 350.degree. F. or greater in a moisture saturated condition. There is therefore a need for resin systems which afford composites which can retain a high level of properties at 350.degree. F. under such moisture saturated conditions.
Most prepreg resins designed for use at 350.degree. F. are made by combining bismaleimides of Formula I with liquid coreactants containing other reactive groups such as amines, epoxides, cyanates or comonomers containing --CH.dbd.CH.sub.2, &gt;C.dbd.CH.sub.2, or --CH.dbd.CH-- groups which can react or polymerize with the carbon-carbon double bonds of the maleimide groups. ##STR1##
In common bismaleimides, R is the residue of an aromatic diamine such as methylene dianiline or m-phenylene diamine. However, neither of these aromatic bismaleimides has adequate solubility in typical liquid coreactants to be useful in a prepreg resin formulation. Previous modifications to improve the processability of such aromatic bismaleimides have included the use of lower melting, more soluble blends of one or more aromatic bismaleimides with aliphatic bismaleimides. The solubility of aromatic bismaleimides has also been improved by the use of various diamines to extend such bismaleimides in a Michael addition reaction to give a mixture of bismaleimides.
The aromatic bismaleimide of formula I, wherein R has the formula ##STR2## or oligomeric versions such as ##STR3## wherein R is the residuum of a dihydric phenol, have also been disclosed and are described in U.S. Pat. No. 3,839,287. A disadvantage of these bismaleimides, however, is again their poor solubility in a various liquid coreactants.
There is a need, therefore, for aromatic bismaleimides with improved solubility with liquid coreactants which can be used in heat resistant resin formulations for structural composites.